The invention relates generally to superconducting magnets, and more particularly to a low-AC loss cold mass structure for superconducting magnets and a process for manufacturing the cold mass structure.
A number of applications exist for superconducting magnets. For example, magnetic resonance imaging (MRI) systems utilize superconducting magnets to generate a strong, uniform magnetic field within which a patient or other subject is placed. Magnetic gradient coils and radio-frequency transmit and receive coils then influence gyromagnetic materials in the subject to provoke signals that can be used to form useful images. Other systems that use such coils include spectroscopy systems, magnetic energy storage systems, and superconducting generators.
In many superconducting magnet assemblies, a superconducting magnet is disposed in a vacuum vessel that insulates the magnet from the environment during operation. The vacuum vessel of MRI and similar magnets is generally made of components that are welded together during assembly of the magnet to form a pressure boundary. The function of the vacuum vessel of an MRI magnet is to provide a reliable pressure boundary for maintaining proper vacuum operation. Vacuum vessels known in the art are usually made of metals such as stainless steel, carbon steel and aluminum. Although, metal vacuum vessels are strong enough to resist vacuum forces, they generate eddy currents and unwanted field distortions in the imaging volume when exposed to an AC field.
The cold mass of a conventional superconducting magnet consists of one or several superconducting coils, a coil support structure and a helium vessel. The helium vessel is a pressure vessel located within the vacuum vessel for thermal isolation. Typically, liquid helium in the helium vessel provides cooling for the coils and maintains the cold mass at a temperature of around 4.2 Kelvin, for superconducting operation. The coils themselves are wrapped around the coil support structure.
Metals, such as stainless steel or aluminum, are usually used to make the helium vessel. When the magnet is operated in an AC field environment, eddy currents will be induced in those metal components, generating AC losses. The AC losses add to the total heat load for the refrigeration system because the eddy currents generate heat at cryogenic temperatures, which is expensive to remove. For certain superconducting magnet applications, these AC losses can be significant and requires to be minimized or eliminated if possible.
Thus, there is a need for reducing field effect losses from eddy currents, while providing desired cooling for superconducting magnets.